The Collaborative International Dictionary of English v.0.48:

Commandant \Com`man*dant"\, n. [F., orig. p. pr. of commander.]
   A commander; the commanding officer of a place, or of a body
   of men; as, the commandant of a navy-yard.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commandant
    n 1: an officer in command of a military unit [syn: commanding
         officer, commandant, commander]